Process for wet spinning aromatic polyamides



Feb. 26, 1963 F. w. KING 3,079,219

PROCESS FOR WET SPINNING AROMATIC POLYAMIDES Filed Dec. 6, 1960 INVENTOR FRANK W. KING ATTORNEY Patented Feb. 25, 1963 nice smears was SllNNlNG Anon-ratio rorvarvnnns Frank W. King, New astle, Del, assignor to E. i. du Pont tie Nemours and Company, Wilmington, Del, a corporation of Eelawarc Filed Dec. 6, 11960, Ser. No. 74,143 3 Claims. (Ci. lit-5 This invention relates to novel attenuated structures such as synthetic fibers, filaments, yarns and films or wholly aromatic polyarnides and to a novel process for producing the same.

Wholly aromatic polyamides and shaped structures thereof exhibiting a valuable combination of properties are described in Hill et al. US. Patent No. 3,006,399, issued October 31, 1961, and Beste et al. US. Patent No. 3,068,188, issued December 11, 1962, including the production of high melting, highly crystalline shaped structures with high tenacity. However, attenuated structures with even higher tenacities than heretofore thought possible are provided by the present invention.

It is an object of this invention to provide attenuated structures of wholly aromatic polyamides that are capable of withstanding a greater load than heretofore when stretched in contact with a surface heated to at least 300 C. Another object of this invention is to provide highly crystalline drawn structures of wholly aromatic polyamides that have higher tenacities at moderate temperatures than any produced heretofore. A further object of this invention is to provide a novel wet extrusion process that enables solvent to be readily extracted from the formed structure and permits drawing of the structure to be immediately and continuously carried out before, during or after solvent extraction. Still another obiect of the invention is to provide a novel coagulating bath for wet spinning aromatic polyarnides. ther objects will be apparent from the description that follows.

In accordance with this invention, an outstanding improvement in the wet spinning of wholly aromatic linear polyamides is provided by the use of an aqueous coagulating bath containing at least about 40% of calcium thiocyanate. The bath very quickly coagulates solutions of these polyamides in a manner which not only facilitates the spinning operation itself, but also makes possible the production of filaments of greatly increased strength. Minor amounts of the polyamide solvent used in the spinning solution do not interfere with the ellicicncy of this coagulating bath and may be allowed to accumulate in the bath up to a concentration of 25%.

The coagulating bath of this invention is particularly useful in the spinning of linear polyamides, wherein the amides directly connect wholly aromatic groups, from solution in a low molecular weight dialltyl amide containing a metal chloride such as calcium, magnesium, strontium, barium, lithium or aluminum chloride. The coagulated filaments or other attenuated structure produced in this way, after removal of solvent and metal chloride by aqueous extraction, can be drawn at a tension of at least 1.5 gram per denier for at least one second in contact with a surface heated to 300 C. or higher. The resulting hot drawn filaments are highly crystalline structures having tenacities in excess or" 7.5 g.p.d. (grams per denier) when prepared from polyamide having an inherent viscosity of at least 1.9 subjected to a total draw in excess of 4 times the initially formed coagulated filament length. A preferred process provides tenacities in excess of 8.0 g.p.d., measured at 65% relative humidity and 70 F. using a rate of elongation of 60% per minute.

The polymer solution of this invention is comprised of about 15 to 25% of a wholly aromatic polyamide as described in aforementioned US. Patents No. 3,006,899,

and No. 3,068,188, such as poly(metaphenylene isophthalamide) or a poly-3-benzamide more fully described hereinafter, with an inherent viscosity of about 1.0 to 2.5 (when determined in solution in concentrated sulfuric acid at 30 C. at a concentration of 0.5 gram of polymer per cc. of solution), about 0.5 to 3% of magnesium, strontium, barium, lithium or aluminum chloride and a dialkyl amide such as dimethylforrnamide or dimethylacetamide. The solution is preferably extruded at about 15 to 30 C.

The coagulating bath of this invention is comprised of about 40% to 60% or more of calcium thiocyanate [Ca(SCN) preferably at least about 50%, with from 0 to 25% of the low molecular weight dialkyl amide used as a solvent in the polymer solution, and water. Preferably the bath will be used for both coagulation and preliminary extraction of the solvent from the polymer structure and will have a temperature of about 50 to C.

A preferred process for preparing these high tenacity structures comprises extruding the polymer solution into the calcium thiocyanate coagulating bath and thereafter continuously extracting the solvent from the structure while drawing at a speed of from about 1 to 4 times that of the initial draw-01f speed, drying and thereafter passing the dried structure over a surface heated to at least 300 C., and preferably heated to at least 330 C., to crystallize the structure and simultaneously therewith drawing an additional amount at a tension of not less than 1.5 g.p.d. to provide a total draw of at least 4 times.

Up to the present time, aromatic polyamides have preferably been spun by the dry spinning method in order to produce better physical properties in the yarn than has been possible by any wet spinning process heretofore tried. Properties of dry spun aromatic polyamide yarns were at least about 15 or 20% better than any aromatic polyamide wet spun yarn produced prior to this invention, whereas the wet spun yarns of this invention when drawn and crystallized by hot drawing have strengths at least 25% better than have been obtained from dry spun aromatic polyamide yarns.

Aqueous solutions of calcium thiocyanate of about 50% concentration and at temperatures between 100 and C. produce very strong filaments that are considerably stronger than have been obtained by wet spinning in other bath compositions. High strength is developed within an inch of the spinneret face, thus permitting the filaments to be handled easily and rapidly through the coagulation and extraction baths. Because coagulation is rapid, the spinneret orifices may be placed as close together as is practical to make them, and filaments of various shapes according to the spinneret hole shape are readily produced.

The very rapid set-up of a thin skin on the extruded filaments is believed attributable to a stable chemical complex which forms between the polymer solvent and the calcium thiocyanate. By virtue of this complex formation the dialkyl amide solvent at the surface of the filament is immediately tied up and the membrane formed together with the strong attraction of the calcium thiocyanate for water is believed to prevent water penetration into the filament until the amide solvent has difiused out from the interior to leave a dense normally-collapsed filament characterized by a finely crenulated skin surrace.

In the following examples, which illustrate specific embodiments of the invention, parts and percentages are by weight unless otherwise indicated:

EXAMPLE I A wholly aromatic polyamide is prepared by reaction of meta-phenylene diamine with isophthaloyl chloride using as a solvent, dimethylacetamide. The dimethylacetamide is distilled prior to use and kept dry until it is used. The meta-phenylene diamine, 25.92 parts, is placed in a glass vessel equipped with a paddle stirrer, a nitrogen inlet and a drying tube. To this is added 226 parts of distilled dimethylacetamide and the vessel is sweptout with nitrogen to remove atmospheric oxygen from the reaction mixture. A slush of Dry Ice and acetone is placed around the vessel to chill the solution and in this process the solution is frozen to a mush. Then, 48.8 parts of isophthaloyl chloride is added all at once, and the Dry Ice bath is replaced by an ice-water bath. Stirring is continued for about 20 minutes to /2 hour. At this point, a very stirrable mass results.

There is an excess of dimethylacetamide hydrochloride .above what is. soluble in the dimethylacetamide in this reaction mixture, and as a result, some of this amide salt is dispersed rather than in solution. About half the cal- .culated amount of amide salt separates. The solution contains approximately twenty percent of polymer based on dimethylacetamide and a considerable amount of hydrochloric acid resulting from the chemical interchange in the form of the acid salt of dimethylacetamide which is substantially neutralized by the addition of ammoniain an amount equal to 90% of the I-ICl present. This results in the immediate separation of ammonium chloride which is insoluble in dimethylacetamide and which is readily removed by filtration. The residual hydrochloride is then neutralized to a pH of about 6.0 by the addition of calcium hydroxide and this results in the formation of soluble calcium chloride, which is desirable for increasing the thermal stability of the solution. The resulting solution contains about 18.0% polymer of 1.91 inherent viscosity and 0.88% calcium chloride.

This solution is spun through a 100 hole spinneret, each hole being circular and 0.003 inch in diameter, into an aqueous bath of 50% calcium thiocyanate and dimethylacetamide maintained at a temperature of 70 C. 'After a bath travel of 12 inches, the yarn is taken over a set of take-up rolls at 25 y.p.m. (yards per minute) and then is drawn 2.5x by passage onto a driven windup bobbin operated at 67.5 y.p.rn. surface speed. The drawn yarn is washed free of bath components and residual solvent by immersion in a tank of water and is thereafter again drawn at a tension in excess of 1.5 g..p.d. over a plate heated to 330 C. between pairs of driven rolls at an additional draw ratio of 1.6x giving a total draw ratio of 4.0x, This yarn is of good appearance and luster and the physical properties of the filaments are as EXAMPLE II A solution of poly(meta-phenylene isophthalamide) in dimethylacetamide containing calcium chloride as a solution stabilizer was prepared by dissolving 540 parts of purified meta-phenylene diamine in 5430 parts of redistilled dimethylacetamide contained in a glass vessel equipped with a stirrer and maintained under an atmosphere of nitrogen. The solution was cooled to 30 C. and 964' parts of isophthaloyl chloride was added while keeping the temperature below 5 C. After thirty minutes of stirring, 153 parts of ammonia was added at -.-5 C. to precipitate most of the hydrochloric acid (present as dimethylacetamide hydrochloride complex) and the solution centrifuged to remove ammonium chloride crystals. The supernatent liquid of low molecular weight polymer containing about 5% of the initial primary amine groups was returned to the reaction vessel and at atemperature of about 3045 C. successive additions of isophthaloyl chloride were made over about thirty minutes. These successive additions were in decreasing amounts (36 parts, 5.5 parts, 3.5 parts, 1.2 parts until a suitably viscous solution resulted indicating high molecular weight of polymer. An addition of 49.0 parts of isophthaloyl chloride was rthus added to the 964 parts of originally added isophthaloyl chloride making a total addition of 1013 parts. Finally, 34 parts of calcium hydroxide slurried in 212 parts of dimethylacetamide was added to complete neutralization to a pH of 6.5. The viscous solution containingabout 21% polymer having 2.11 inherent viscosity also contained 0.92% calcium chloride.

The solution was spun through a l00-hole spinneret as in Example I into an aqueous bath of 60% calcium thiocyanate containing initially no dimethylacetamide and at a temperature of to C. The yarns after fifteen inches bath travel were passed over a take-up roll having a surface speed as shown in Table A below and were then passed successively into a first washing bath containing water with 'a contact time of thirty seconds at the temperatures shown in Table A and then through a second washing bath at 20 C. with ten seconds contact time. The yarns were drawn between the initial drawotf roll and the washing bath rolls as shown in the table and were then wound up on bobbins. The wet yarns were dried and further drawn immediately after drying to give the total draw shown in Table A, by passing them over a plate heated to 300 C. under a tension of not less than 1.5 g.p.d. The yarn properties are shown in Table A.

EXAMPLE III A solution of 2280 parts of meta-phenylene diarnine dissolved in 18,100 parts of dimethylacetamide was prepared in a glass-lined jacketed vessel equipped with a propeller type mixer and in an atmosphere of nitrogen. At about room temperature 4071 parts of molten isophthaloyl chloride was added with stirring during twenty minutes and the stirring continued for one hour. At the end of this time, 646 parts of ammonia was added over a period of about one hour and the solution filtered under nitrogen in a centrifugal filter using a laid asbestos filter to remove the ammonium chloride. The filtered solution was run into a nitrogen filled vessel with a suitable spiral stirrer and additional isophthaloyl chloride was added as follows: parts at the rate of 12 parts per minute followed by 62 parts at the rate of 0.4 part per minute. By the time all the isophthaloyl chloride had been added, maximum viscosity was attained and the solution was neutralized to a pH of 6.0 with calcium oxide slurried in dimethylacetamide. The solution was found to contain 19.6% polymer of 1.93 intrinsic viscosity and 1.99% calcium chloride. The solution had a viscosity of 179 poises at 100 C.

The drawing is a diagrammatic showing of the spinning, preliminary drawing, and washing arrangement. The polymer solution was forced from the hopper 10 through a Zenith gear-type metering pump 12 and then through a 6-inch diameter plate filter 14. The filter medium consisted of a one-inch depth of to 200 mesh sand, seven layers of calendered cloth woven from high bulk poly(ethylene terephthalate) fiber yarn, two sheets of line filter paper sandwiched between layers of the cloth and finally a felt of poly(ethylene terephthalate) fiber at the top or outlet side of the filter. The spinneret 16.was A; inch in diameter, of stainless steel and contained 100 circular holes, each 0.0024 inch diameter. A spinneret filter was provided composed of two 80-mesh platinum screens, three layers of. the calendered cloth just previously described, and one layer of poly(ethylene terephthalate) fiber felt. The calcium thiocyanate coagulating and extraction bath 18 contained 55.0% calcium thiocyanate, 11.7% dimethylacetamide and 0.5% calcium chloride, the balance being water, and was maintained at a temperature of 125 C. To remove low molecular weight polymer extracted from the filaments, the bath was constantly filtered and recirculated by means of pump 20 and glass fiber filter 22.

The rollers 24, 26 and 28 were so arranged, each with an auxiliary guide roll, 30, 32 and 34 respectively, to enable a plurality of passes of yarn to be made around each of the rollers. With the surface speed of the drawofi" roll 24 at 8 y.p.m., and with a relative short bath travel, the initial contact time of the freshly-formed filaments withthe coagulating bath was about 1 to 2 seconds. The yarn made passes around the roll 24 and its auxiliary guide roll 30 in the initial extraction step and the contact time of the yarn with this bath 36 was about 2 minutes. In the first water extraction at C. in bath 38 the yarn made 15 passes around roll 26 and its auxiliary roll 32 for a contact time of about two minutes. In the second water wash at 95 C. in bath 40 the yarn made six passes around roll 28 and its auxiliary roll 34 so that the contact time of the yarn with this bath was about /2 minute. tained at a surface speed of 28 y.p.m. thereby giving a draw ratio between roller 24 and the wind-up bobbin of 3.5

Several'samples of yarn collected on the wind-up bobbins were subsequently dried by unwinding and passing the yarn over a heated surface at a temperature somewhat in excess of 100 C. Subsequently, the yarns were further drawn to a total draw ratio as indicated below in Table B while the yarns were passing over a surface heated to 330 C. to crystallize the filament structures.

In each case the draw ratio and conditions of drawing were so selected that the yarn tension during this passage was never less than 1.5 -g.p.d., being in all cases above about 1.8 or 1.9 g.p.d. Also in Table B are given tenacities, elongations, initial moduli and deniers of these several yarn samples. The tenacity is measured at 65% relative humidity and 70 F. on an instron tester at 60% per minute elongation. From this test is obtained the tensile strength in grams per denier, the elongation in percentage and the initial modulus in grams per denier. It will be noted that the denier of the 100 filament structure varies quite widely due primarily to changes in pump delivery and to minor differences in total draw ratios.

Drawing during spinning can be varied widely without affecting physical properties substantially. This is illustrated by the following table of results which indicates variations in draw between l.0 and 3.0x during spinning have a relatively small effect on the yarn properties when the total amount of draw is of the same order.

The wind-up bobbin 42 was main- Table C Draw during Total Tenacity Elonga- Initial Denier spinning draw tion modulus 3. 0 4. 2 7. 6 17 2. 5 2.0 3. 6 6. 4 22 104 2. 7 l. 0 3. S 6. 8 22 9B 3. 0

From the examples it will be noted that the best results are obtained when the concentration of calcium thiocyanate in the spinning is high, i.e., above 50% and even as high as 55% or 60%. Spinning bath concentrations of calcium thiocyanate of at least about 50% are therefore preferred. The amount of amide solvent in the coagulating and extraction baths may be varied widely and starting with a fresh bath it may be zero initially. Practically, however, it is better to operate so that a small relatively constant value is maintained, such as 10 to 15%, although as much as 25% may be tolerated without any ill effects. For the production of very high tenacity yarns in excess of 8 g.p.d., it is essential that the polymer solution used be prepared so as to minimize the formation of gels due to very high molecular weight material. To obtain such solutions, the final isophthaloyl chloride additions must be made slowly with adequate stirring as indicated above in Example III. When such polymer solutions are spun into yarns, the filaments are more uniform and are capable of withstanding higher tensions at the time of crystallization and final hot drawing. In order to attain the very high tenacities frequently desired, the yarn should be capable of withstanding for at least one second a tension of at least 1.5 g.p.d. while it is being drawn in contact with a surface heated to 300 C. and preferably to at least 330 C.

Wholly aromatic polyamides and polymer solution compositions other than shown in the examples may also be used in accordance with this invention with good results. The US. patents mentioned earlier list numerous other aromttic polyamides in other amide solvents and with other metal halides present as stabilizing agents. Poly- 3-benzamide, a linear polyamide comprised of recurring aminobenzoyl units of the structural formula:

The yarns, filaments, fibers and the like produced in accordance with this invention have the necessary high strength and high modulus to make them especially suitable for use as cord for reinforcing rubber products, such as tires, belts, hose and the like, as a reinforcing structure in plastic goods, as a sewing thread, and for any use under high temperature conditions where high strength per unit of weight is needed.

Since many different embodiments of the invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited by the specific illustrations except to the extent defined in the following claims.

I claim:

1. In the wet spinning process for forming shaped articles from a solution of a linear polyamide wherein the amide linkages directly connect wholly aromatic groups,

the improvement which comprises extruding said solution into an aqueous coagulatingbath-containing at least about 40% of calcium thiocyana'te.

2. Aprocess as defined in claim 1 wherein the aqueous coagulating bath contains about 40% to about 60% of calcium thiocyanate and O to 25% of a low molecular weight dialkyl amide.

3. A process as defined in claim 2 wherein the aqueous coagulating bath is at a-ternperature-of 50 to 130 C.

4. The process for forming attenuated shaped articles from a linear polyamide wherein the amide linkages directly connect Wholly aromatic .groups, 'said polyamide having an inherent viscosity of about 1.0 to 2.5, which comprises preparing a solution of said :polyamide in a low molecular weight dialkyl amide solvent to contain about to25% of'the polyamide and about 0.5% to 3% of a metal chloride selected from the group'consisting of'calcium, magnesium, strontium, barium, lithiuni and aluminum chlorides, extruding said solution in the form of :a continuous attenuated structure, coagulating said solution to form a self-supporting structurein an aqueous bath containingabout 40% to 60% of calcium thio'cyana'te-and up =10 of lowmolecular weightidialkyl amide, removing dialkyl amide solvent and metal chloride from the polyamide by aqueous'extra'ction, and drawing the structure 'over a surface heatedLto at least -0 C. at a tension of at least 1.5 .grams per denier to produce a high melting, crystalline, drawn structure.

5. The process as defined in claim 4 wherein the polyatnide solution is extruded at a temperature of about 215 to30 0. into said bath.

6. The process as defined in claim '4 wherein the polyamide solution is extruded through a spinneret directly into said bath and the bathismaintained at a temperature of about 50 to 130 C.

' '7. Thepro'ces's'jfor forming attenuated shaped articles from a linear poly amide wherein the amide linkages directly connect wholly aromatic groups, said .polyarnide having an inherent viscosity of about 1.0 to 2.5, which comprises preparing a solution of said poly'ainide-in a low molecular weight d'ialkyl amide solvent to contain about 15% to 25% of the polyamide and about 0.5% to 3% of a metal chlcr ide selected from the, group consisting of calcium, magnesium, strontium, barium, lithium and a1ur'ni'nu'm chlorides, spinning said solution at a temperature of about 15 to 30 C. :into anaqueous calcium thiocyanate bath at a temperature of about to 130 C. containing about 40% to 'of calcium thiocyanate and up to 25% of low'molecular weightdialkyl amide to coagulate the solution into filaments, continuously withdrawing the filaments from the coagulation zone and removing remaining solvent by aqueous extraction While forwarding-the filaments iatia speed "within .therange of to about 4 times the initial draw- 0E speed, and drawing the filamentsat ateusion'of at least 1.5 gramsper denier over a surfaceheated to at least 300 C. to crystallize the :polyam'ide structure.

8. The processes defined in claim 7 wherein saidipolyamide has an inherent viscosity .of at least 1:9 and the length-of the filaments withdrawn from the coagulation zone is subsequently increased by a ttotaLdr-aw in excess of 4 times Ito provide a tenacityin excess of 7.5 gramstper denier.

References Cited in the iileo'f this patent UNITED STATES PATENTS 2,130348 Ca rother's a Sept. 20, 193-8 2,212,772 Graves Aug. 27, 1940 2,252,554 Qa'rotherss Aug. 12, 1941 2,277,125 Martin Mar. 24, 1942 2,296,202 Hardy -n Sept. '15, 1942 2,389,655 'Wende e Nov. 27, 1945 2,517,694 :Merion ...-A\1g. 8, 1950 2,625,536 Kirby Jan. 13, 1953 2,715,763 lvlarley Aug. 23,, 1955 2,719,073 Olson ;Sept.,27., 1955 2,768,057 Friederich Oct. 23, 1956 2,9 18,347 Notar-bartolo Dec. 22, .1959

FOREIGN PATENTS 7.45.029 Germany s Feb. 23, 1944 614,625 Great Britain Dec. 30, 1948 

1. IN THE WET SPINNING PROCESS FOR FORMING SHAPED ARTICLES FROM A SOLUTION OF LINEAR POLYAMIDE WHEREIN THE AMIDE LINKAGES DIRECTLY CONNECT WHOLLY AROMATIC GROUPS, THE IMPROVEMENT WHICH COMPRISES EXTRUDING SAID SOLUTION INTO AN AQUEOUS COAGULATING BATH CONTAINING AT LEAST ABOUT 40% OF CALCUIM THIOCYANATE. 